<?php 
$dateBegin = new Zend_Date();
$dateEnd   = clone $dateBegin;

$writer = new XMLWriter();
$writer->openMemory();
$writer->startDocument('1.0');
$writer->startElement('reservations');
foreach ($this->reservations as $reservation) {
    $dateBegin->set($reservation['date_begin'], 'YYYY-MM-DD HH:mm:ss', 'fr_FR');
    $dateEnd->set($reservation['date_end'], 'YYYY-MM-DD HH:mm:ss', 'fr_FR');
    $writer->startElement('reservation');
    $writer->writeAttribute('id', $reservation['id_reservation']);
    $writer->writeElement('room', $reservation['room']);
    $writer->writeElement('date_begin', $dateBegin->toString('dd MMMM à HH:mm'));
    $writer->writeElement('date_end', $dateEnd->toString('dd MMMM à HH:mm'));
    $writer->writeElement('usage', $reservation['usage']);
    $writer->writeElement('creator', $reservation['user']);
    $writer->endElement();
}
$writer->endElement();
$writer->endDocument();
echo $writer->flush();